Typical Bragg cell modulators utilize a surface acoustical wave (SAW) transducer which is positioned on the surface of the material which conducts the acoustical waves that interact with various frequency light wave signals that pass through the device are deflected at their Bragg angle due to the interaction the light frequency waves with the surface acoustical waves. In conventional acousto-optic Bragg cells, whether using surface waves or bulk acoustical waves, (BAW) there is a limitation on the achievable dynamic range due to the generation of intermodulation products. If two signals f.sub.1 and f.sub.2 are present simultaneously there will be inter mod acoustic product signals at 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1. In order to maximize the Bragg efficiency and thus the dynamic range, it is desirable to use high power levels in the acoustic signal, but this increases the intermodulation signal levels. (For example, if f.sub.1 and f.sub.2 are increased by 10 dB, the intermodulation product 2f.sub.1 -f.sub.2 increases by 30 dB.)
Thus, low acoustic power and dynamic range is limited by noise and at high acoustic power by intermodulation products.
The SAW/BAW device of the present invention utilizes a surface transducer of a specially constructed form. The transducer has hyperbolically tapered fingers and is constructed in a manner similar to that shown in U.S. Pat. No. 4,635,008, which was invented by the inventor of the present invention, and is assigned to the assignee of the present invention. This transducer has a number of hyperbolically tapered interdigital fingers which are separated from each other in such a manner that the frequencies launched by the transducer or are received by the transducer, vary from high frequencies at one end of the transducer where the electrode fingers are closely spaced together, to lower frequencies at the opposite end of the transducer where the fingers are more widely separated. The surface acoustical waves that are, therefore, launched by this transducer vary in a like manner as they travel across the surface of the Bragg cell.
A number of reflecting elements are placed in the paths of the SAW's which are preferably spaced-apart, elongated, thin, rectangular reflector elements. These elements may be formed by deposited metal on the surface of the cell, or they may be grooves in the surface, or they may be formed by other conventional means known to those skilled in the art. The Bragg cell of this invention is several times thicker than a conventional normal SAW Bragg cell. In the typical SAW Bragg cell, the structure is relatively thin in order to minimize the effects of bulk waves. However, in the device of the present invention, the reflecting elements are utilized to deliberately deflect a portion of the energy of the incoming SAW's into BAW's which will traverse the cell at an inclined direction into the bulk of the material. The present invention, thereby, provides wide separation of the various frequency components of the impinging light and hence minimizes intermodulation products which limit the dynamic range of present Bragg cells. In addition, SAW to BAW conversion techniques allow for planar construction which is relatively less expensive. The ability of the device to satisfy the Bragg condition over the entire bandwidth increases the useful bandwidth of such devices and increases the efficiency of the Bragg scattering process.